1. Field of the Invention
The present invention generally relates to an internal combustion engine having a control system for controlling the operation of a variable camshaft timing mechanism (VCT) of the type in which the position of a camshaft is circumferentially varied relative to the position of a crankshaft. More specifically, this invention relates to control systems for operating VCT devices in response to fluid under continuous pressure and fluid under pulsation to selectively advance, retard, or maintain the position of the camshaft.
2. Description of the Prior Art
It is known that the performance of an internal combustion engine can be improved by the use of dual overhead camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft-powered chain drive or belt drive. It is also known that the performance of an internal combustion engine having dual overhead camshafts, or but a single camshaft, can be improved by changing the positional relationship of a camshaft relative to the crankshaft.
It is also known that engine performance in an engine having one or more camshafts can be improved by varying camshaft timing, specifically in terms of idle quality, fuel economy, reduced emissions, or increased torque. For example, the camshaft can be xe2x80x9cretardedxe2x80x9d for delayed closing of intake valves at idle for stability purposes and at high engine speed for enhanced output. Likewise, the camshaft can be xe2x80x9cadvancedxe2x80x9d for premature closing of intake valves during mid-range operation to achieve higher volumetric efficiency with correspondingly higher levels of torque. In a dual overhead camshaft engine, retarding or advancing the camshaft is accomplished by changing the positional relationship of one of the camshafts, usually the camshaft that operates the intake valves of the engine, relative to the other camshaft and the crankshaft. Accordingly, retarding or advancing the camshaft varies the timing of the engine in terms of the operation of the intake valves relative to the exhaust valves, or in terms of the operation of the valves relative to the position of the crankshaft.
There are a multitude of VCT architectures using actuating components that include piston-cylinder devices, hub and vanes, single lobe vanes, and opposed lobe vanes. Similarly, there are at least three distinct styles of VCT actuation in the prior art. The first style is referred to hereafter as an Oil Pressure Actuated (OPA) VCT. The OPA system includes a VCT responsive to fluid under continuous pressure generated by an engine oil pump. The second style is referred to hereafter as a Camshaft Torque Actuated (CTA) VCT. The CTA system includes a VCT responsive to fluid under pulsations generated by torque pulses in the camshaft. The third style is referred to hereafter as a multi-mode VCT. The multi-mode system includes a VCT responsive to both fluid under pressure and under pulsation to oscillate the camshaft.
With OPA devices, the VCT uses fluid output of an engine oil pump where the actuation rate of the VCT is limited by the available hydraulic power supplied by the pump. Many such VCT systems incorporate hydraulics including a hub having multiple circumferentially spaced vanes cooperating within an enclosed housing having multiple circumferentially opposed walls. The vanes and the walls cooperate to define multiple fluid chambers, and the vanes divide the chambers into first and second sections. For example, U.S. Pat. No. 4,858,572 (Shirai et al.) teaches use of such a system for adjusting an angular phase difference between an engine crankshaft and an engine camshaft using oil pressure from a pump. Shirai et al. discloses fluid circuits having check valves, a spool valve and springs, and electromechanical valves. Fluid is transferred from the first section to the second section, or vice versa, to thereby oscillate the vanes and hub with respect to the housing in one direction or the other. Each branch of the fluid flow path runs from one section to the other through a drainage clearance between the hub and the camshaft, back through the oil pump, and then through the spool valve and a check valve. The check valve prevents fluid from flowing out of each section back to the spool valve.
With CTA devices, the VCT uses the energy of reactive torques in the camshaft to power the VCT hydraulically through a check-valve fluid circuit. The camshaft is subjected cyclically to resistant torques when the rising profiles of the cam lobes open the valves against the action of the valve springs, and then to driving torques when the valve springs close the valves by causing them to follow along the descending profiles of the cam lobes. The alternating resistant and driving torques in the camshaft translate into slight pulsations in the vane. These pulsations result in alternating pressure differentials across the vane that alternately compress the fluid in the advance and retard fluid chambers. To retard the camshaft, fluid is allowed to escape during the pulsations from the advance chamber and flow to the retard chamber through one branch of a one-way fluid circuit. Alternately, to advance the camshaft, fluid is allowed to escape during the pulsations from the retard chamber to the advance chamber through another branch of a one-way fluid circuit. Accordingly, the VCT changes phase by exchanging fluid from one fluid chamber to the other using the differential in pressure of the fluid in the fluid chambers to increase the volume of one fluid chamber at the expense of the other.
For example, U.S. Pat. 5,645,017 to (Melchior) teaches use of a torque pulse actuated VCT to change phase of a camshaft. The ""017 patent discloses a vane type VCT having a vane within a housing that delimits opposing antagonistic chambers that are interconnected by two unidirectional circuits having opposite flow directions. A valve communicates with the two unidirectional circuits so as to transfer fluid from one antagonistic chamber to the other in response to alternating pressure differentials between the antagonistic chambers, where the pressure differentials result solely from torque pulsations in the camshaft and vane.
In the systems described above, VCT actuation is accomplished in response to torque pulsation in the camshaft or in response to engine oil pressure from an engine oil pump, but not both. This presents a significant disadvantage.
First, there are shortcomings to using only the CTA powered VCT. The CTA device has a significantly lower frequency response than the OPA device, even though the potential actuation rate of the CTA device is substantially higher than the OPA device due to the larger amount of energy in the cam torque inputs. For example, inline four cylinder engines typically operate at relatively high speeds and therefore generate very high frequency torque pulses to which CTA systems do not respond quickly enough to cause actuation of the VCT. Thus, the relatively low frequency response of the CTA system results in a dramatic drop in CTA performance at the higher engine speeds of the inline four cylinder engines. Similarly, inline six cylinder engines typically exhibit low amplitude camshaft torque pulses that are also inadequate to actuate the VCT.
In contrast, the OPA systems have nearly the opposite problem. Since the actuation rate of the OPA device is strongly dependent on engine oil pressure, the device performs well at higher engine speeds, when the oil pump is producing an abundance of oil pressure. At lower engine speeds, however, particularly when the engine is running hot, the performance suffers because the oil pump is producing relatively little oil pressure.
Because the OPA device performs well at high speed and the CTA performs well at lower speeds, it would be advantageous to combine both strategies and architectures into one multi-mode VCT device and be able to selectively switch between the two independently and/or use both simultaneously. For example, U.S. Pat. 5,657,725 (Butterfield et al.), which is assigned to the assignee hereof teaches uses of a dual-mode VCT system to change phase of a camshaft. The ""725 patent discloses a dual-mode device responsive to torque pulses and/or engine oil pump pressure for actuation. In the ""725 patent there is disclosed a VCT apparatus having a vane within a housing that delimits opposing advance and retard chambers that are interconnected by an hydraulic circuit having two check valves and a spool valve therein. Here, fluid flows from one chamber to the other, through one check valve and then through the spool valve, in response to sufficiently strong torque pulsations in the vane. When there are not sufficiently strong pulsations present in the vane, fluid flows from the one chamber, not through the check valve, but directly through the spool valve to exhaust. Simultaneously, make-up fluid from the engine oil pump flows through the spool valve both directly to the other chamber and indirectly to the other chamber, by cycling in parallel through the other check valve back through the spool valve.
While the ""725 patent discloses a significant improvement upon the prior art, there are still some disadvantages. For example, the system is two-position only and is not capable of maintaining position between fully advanced and fully retarded positions. Additionally, the system uses a relatively complicated hydraulic circuit and spool valve system.
Accordingly, what is needed is a multi-mode VCT system that is capable of advancing, retarding, and maintaining a camshaft in intermediate positions over the entire speed range of an engine and uses relatively inexpensive and uncomplicated and hydraulic circuitry and components.
According to the present invention there is provided a multi-mode VCT system that is capable of advancing, retarding, and maintaining a camshaft in intermediate positions over the entire speed range of an engine and uses relatively inexpensive and uncomplicated and hydraulic circuitry and components.
The present invention includes a variable camshaft timing device including a pulse actuating circuit for oscillating the variable camshaft timing device in reaction to fluid under pulsation. A pressure actuating circuit is included for oscillating the variable camshaft timing device in reaction to fluid under pressure. Advance and retard valves are interconnected with the pulse and pressure actuating circuits for independently and simultaneously activating the pulse and pressure actuating circuits. Finally, an exhaust valve is positioned in fluid communication with the pulse and pressure actuating circuits, whereby the variable camshaft timing device may be oscillated using one or both of the pulse actuating and pressure actuating circuits, and may be maintained in position using one or both of the pulse actuating and pressure actuating circuits.
Accordingly, it is an object of the present invention to provide an improved variable camshaft timing device for varying camshaft timing in an internal combustion engine.
It is another object to provide a multi-mode variable camshaft timing device that is capable of operating in response to fluid under pressure from a pump and fluid under pulsations from alternating camshaft torques.
It is yet another object to provide a multi-mode variable camshaft timing device that is capable of maintaining position anywhere between a fully advanced and fully retarded position over the full range of engine speed, and does not necessarily require use of a spool valve, but may as an option.
These objects and other features, aspects, and advantages of this invention will be more apparent after a reading of the following detailed description, appended claims, and accompanying drawings.